The invention relates to devices for generating magnetic fields.
Magnetic recording requires relatively high magnetic fields to be produced locally in order to fix the orientation of small magnetic domains on the surface of a magnetic disk. The demand is for ever-increased surface density coupled with stability of the magnetic domains, so once data is written it does not change. These requirements effectively mean that the ideal magnetic material should have high coercivity. However, the higher the coercivity, the greater the magnetic field required to set the domain orientation and hence write a bit of data.
Longitudinal magnetic recording (LMR) is the conventional approach for recording data on magnetic discs. Magnetic domains are aligned with the plane of the magnetic disc and the writing element is formed by the open poles of a ring magnet arranged sideways above the recording layer, referred to as a “ring” writing element. Longitudinal recording has an estimated density limit of 100-200 Gbits per square inch. The super-paramagnetic effect is performance limiting. Stronger magnetic fields are required to record data on materials with high coercivity and small domain size.
Perpendicular magnetic recording (PMR) is a more recently developed approach for recording data on magnetic discs. Magnetic domains in the recording layer are aligned perpendicular to the plane of the magnetic disc. An additional soft magnetic layer is arranged underneath the recording layer and cooperates with a monopole writing element to direct magnetic field directly down through the recording layer. PMR can produce higher data densities than LMR, perhaps by a factor of 5.
Because magnetic field generators are at their limit, an enhancement that has been proposed for magnetic recording is so-called heat assisted magnetic recording (HAMR). HAMR exploits the fact that the coercivity of a magnetic material and hence its switching threshold reduces with increasing temperature, so by heating the disc locally in the region where the data is to be written, the magnetic field required to effect writing is reduced. Local heating can be provided by a focused laser beam. In 2009, HAMR devices achieved a density of 250 Gbits per square inch. It is predicted that data storage densities of 1 terabit per square inch or more will be achievable with HAMR. Mainstream commercial adoption of HAMR is expected by 2015.